SKELETAL ISOMERIZATION OF N-BUTENES - II - COMPOSITION, MODE OF FORMATION, AND INFLUENCE OF COKE DEPOSITS ON THE REACTION-MECHANISM

On a fresh HFER zeolite (Si/Al=13.8) at 623 K, the skeletal isomerizat ion of n-butenes is accompanied by various reactions: disproportionati on into propene and pentenes, coking, hydrogen transfer (in the decrea sing order of significance). While there is a rapid deactivation of di sproportionation, coking, and hydrogen transfer reactions, owing to th e blockage of the pores by carbonaceous compounds (coke), an increase in the rate of isomerization is at first observed followed by a decrea se at: long time-on-stream. At the maximum of isomerization only 10% o f the pore volume remains accessible to nitrogen adsorbent. Moreover, IR spectroscopy shows that 75% of the OH groups are in interaction wit h coke molecules, the remaining 25% being inaccessible to ammonia and, hence, to the reactant, Coke was found to be composed of slightly con densed aromatics (with two to four aromatic rings) trapped at the inte rsections of the eight-and 10-membered-ring channels. Coke molecules r esult from the transformation of secondary benzenic products, slowly d esorbed from tile zeolite pores, through two different ways, (i) the c lassical route via alkylation, cyclization, and hydrogen transfer step s and (ii) dehydrogenative coupling. The large significance of this la tter reaction in the formation of HFER coke molecules in comparison to the other zeolites, can be related to the proximity of adjacent chann el intersections, To explain the initial increase in the rate of buten e isomerization a new reaction process is proposed involving as active sites benzylic carbocations formed from coke molecules trapped in por es located near the outer surface of the crystallites (pore mouth cata lysis). The deactivation of this reaction which occurs at long time-on -stream is shown to be due to the growth of the active coke molecules, (C) 1998 Academic Press.